In a catalyst apparatus for purifying exhaust gas discharged from an internal combustion engine of an automobile or the like, various catalysts have been used in response to object thereof. As a major catalyst component thereof, there is platinum group metals, and usually, it is used by being supported, in high dispersion, onto a refractory inorganic oxide having high surface area of activated alumina or the like (refer to PATENT LITERATURE 1).
As the platinum group metals as the catalyst component, platinum (Pt), palladium (Pd), and rhodium (Rh) have been known, which have been used widely as the catalyst for purification of exhaust gas discharged from an internal combustion engine of an automobile or the like. In the aforementioned TWC, a catalyst activated specie superior in oxidation activity, of Pt, Pd or the like, and Rh superior in purification activity of NOx are used in combination, in many cases. In recent years, regulations on hazardous substances contained in exhaust gas, in particular, NOx, have become more and more severe. Accordingly, it is necessary to effectively use Rh superior in purification activity of NOx. In addition, Rh is scarce in output and high price, which has caused price hike in recent market. Therefore, it is preferable to decrease used amount of Rh, as a catalyst activated specie, in view of resource protection as well as cost.
In addition, as for the exhaust gas purification catalyst, to attain enhancement of further purification performance, the addition of various promoter components, in addition to platinum group metals, to the catalyst has been investigated. As these promoter components, an Oxygen Storage Component (OSC), or an alkaline earth metal, or zirconium oxide, zeolite and the like have been known.
Among these, OSC is the one which stores and discharges oxygen in exhaust gas, and cerium oxide has been known. Cerium oxide stores oxygen as CeO2, when oxygen concentration is high in exhaust gas, and discharges oxygen by being converted to Ce2O3, when oxygen concentration is low. Oxygen discharged is activated oxygen, which promotes purification of HC and CO, by being utilized in oxidation action by Pt or Pd. In addition, OSC also acts to buffer oxygen concentration change in exhaust gas, by storage and discharge of oxygen. By this action, purification performance of exhaust gas is enhanced in TWC. TWC is the one which performs oxidation and reduction by one catalyst, and has a range of exhaust gas components suitable for purification in view of designing. This range depends on air; fuel ratio in many cases. Such a range is called a window, and in many cases, exhaust gas combusted at the neighborhood of theoretical air-fuel ratio, called Stoichiometry, is set as the window region. Because change in oxygen concentration in exhaust gas is being buffered, this window region can be maintained for a long period of time, and purification of exhaust gas can be performed effectively. This is said to influence particularly on purification characteristics of NOx by Rh.
As such a cerium oxide, a pure cerium oxide can also be used, however, it is often used as a composite oxide with zirconium (refer to PATENT LITERATURE 2). A cerium-zirconium composite oxide is said to have high heat resistance and high storage and discharge rate of oxygen. The reason is considered that a crystal structure of the cerium-zirconium composite oxide is stable, and does not inhibit the action of a cerium oxide, which is a main OSC component, thus functions as the OSC up to inside of the particle.
On the other hand, in purification of NOx by Rh, it is considered that, for example, a steam reforming reaction or a CO+NO reaction is promoted via the Rh component as shown below and to purify NOx.HC+H2O→COx+H2  (1)H2+NOx→N2+H2O  (2)CO+NO→CO2+½N2  (3)
And, it has become known technology that the zirconium oxide promotes the steam reforming reaction or the CO+NO reaction, when used together with the Rh component (refer to PATENT LITERATURE 3).
In addition to a purification reaction of NOx by HC, shown below, such reactions are generated, and thus, purification of NOx is further accelerated, therefore, presence of a promoter for accelerating the steam reforming reaction, other than the zirconium oxide, is very important.NOx+HC→CO2+H2O+N2  (4)
As the promoter component, in addition to this, an alkaline earth metal such as the Ba component has also been known (refer to PATENT LITERATURE 4). The Ba component temporarily stores NOx contained in exhaust gas, and purifies the stored NOx by reducing to N2 by a reducing component contained in exhaust gas.
In general, NOx is generated in a large quantity, when fuel supplied to an engine is less, or amount of air is more, or combustion temperature is high. The Ba component temporarily absorbs NOx generated in this way.
And, NOx absorbed into the Ba component is discharged from the Ba component, when NOx concentration in exhaust gas becomes low, and CO concentration in exhaust gas becomes high. In continuing the aforementioned example, this is caused by reaction of Ba(NO3)2 with CO to be converted to BaCO3, and can be said chemical equilibrium. NOx discharged from the Ba component, as described above, is purified by reduction, by a reaction with a reducing component at the Rh component surface.
Such a promoter component may be used in combination of two or more and, for example, TWC has been known where the Ba component and cerium oxide are used (refer to PATENT LITERATURE 5). However, it has been reported that purification performance may be decreased depending on combination of catalyst materials and, for example, presence of the Rh component and the Ba component in the same composition decreases purification performance of NOx (refer to PATENT LITERATURE 6). The reason for this is considered that, because the alkaline earth metal component has action of storing NOx, purification action of NOx in the Rh component is interfered, or an oxidized Rh structure is stabilized by electron donating action from Ba to Rh.
Therefore, it has been proposed to enhance purification performance of NOx and heat resistance, by supporting the Rh component and the Ba component onto alumina in a separated state (refer to PATENT LITERATURE 7). In this LITERATURE, there is no description on what degree the Rh component and the Ba component are separated in the catalyst layer, however, in the case of using water-soluble Ba acetate as a Ba source, the Ba component elutes into slurry, and thus it cannot be said that it is sufficiently separated from the Rh component. As a result, the Rh component and the Ba component come close and thus a problem of decrease in purification performance of NOx cannot be solved sufficiently.
In addition, there have been challenged a method for adding a rare earth oxide, such as neodymium oxide, praseodymium oxide, to alumina, in order to enhance heat resistance of alumina, which is a base material, or CeO2, which is an OSC component (refer to PATENT LITERATURE 8), or adding a rare earth oxide such as lanthanum oxide, neodymium oxide, to a cerium oxide (refer to PATENT LITERATURE 9), as well as supporting Rh, Pd or the like, which is a noble metal, onto both of alumina particles having heat resistance enhanced by lanthanum oxide, zirconia or the like, and CeO2 fine particles having heat resistance enhanced by neodymium oxide, zirconia or the like (refer to PATENT LITERATURE 10).
In this way, there are various combinations of the catalyst components, and complicated reaction routes are taken by interaction of the catalyst components, and thus by overall investigation on these, a combination of the catalyst components which exerts purification action most has been searched.
By the way, the exhaust gas purification catalyst may be arranged as one set in exhaust gas passage, however, there may be the case where two or more are arranged. This aims at more utilization of characteristics of the exhaust gas purification catalyst, in association with strengthening of exhaust gas regulations, and each optimum position is required to be set, in response to durability (heat resistance, atmosphere resistance, poisoning resistance), catalyst characteristics (oxidation activity, reduction activity) or the like which each noble metal of platinum, palladium and rhodium has.
In addition, to reduce used amount of high-priced noble metals or rare earth metals leads to efficient utilization of a limited resource, and also in view of this, it has been required to set the exhaust gas purification catalyst at the optimum position of exhaust gas passage, in response to respective characteristics of the noble metals or the rare earth metals.
Still more, regulations on exhaust gas has become severer and severer in recent years, and advent of such a catalyst has been desired that exerts more superior exhaust gas purification performance, by using multiple catalysts. Among exhaust gas, regulation value for, in particular, NOx has become severer, which cannot be attained by the above conventional catalysts, and thus also in TWC, needs of the catalyst for exhaust gas purification superior in purification performance of NOx has increased.